Well ... Okay. but Thanks anyway TL
Like it or not.
Your assistance has been invaluable. Much of the guesswork has been taken out of the project.

Here's where I'm at now:
* Full New crossover kits are in the air to me now from Jerry (Falcon Acoustics). I opted for the revised design using the T-27 aB circuit and the steeper 18dB MF-HF slope.
* I'm pretty sure I'll be ordering the wool stuffing from QTA Systems in Cambridge UK. I can get the quantity I need at a better price than any alternate type here in the Great White Northern Wilds. (Mr. Adams of QTA is a pleasant and humorous man. I heartily recommend dealing with him!)
* I will install terminal plates on the rear, with 5 way binding posts.
* I'm in the process now of braiding Cat5 cable (12 double twisted lengths of 24 gauge wire) in sufficient lengths to connect the input to the crossover. This necessitates drilling holes and threading it through the 2 mid-range cavity sectional lumber. This is easiest however as I don't relish the idea of trying to hack out the cement-like sealant in the baffle wire channel!

Everything else stays original aside from the new wool stuffing behind the woofer and B-110.
Once one side is complete there will be extensive mono listening (tweaking of stuffing if and as required), and comparison made to the currently used IMF 80's.
The steps after that will be up to SHE, Ms. Golden Ears, Resident sound expert / musician, who will ultimately decide if the TL-200's make the grade for main system use.
If they do ... It's a complete cabinet restoration ahead. Stripped, veneer repair (minor), refinished to compliment the decor, and living happily ever after

Interesting that no one has yet opined about a comparison between the R-50's - TL-200's and the IMF's! I'm very interested in that. The only major driver difference is IMF's Celestion HF-1300 tweeter in place of Kef's T-27._________________Music is food for the soul.

My crossover kits have arrived.
I am pleased to note that the instructions included by Falcon indicate it's a simple matter to set these up for bi-wiring.

Since it adds little to the cost and I was planning surgery on the rear panel for the installation of binding post plates anyway, I have ordered up a dual input set.
Going a step further, since I'm making that extra effort I thought I'll also separate the bass section filter components (only 1 inductor and single cap) and mount them on a second board and locate this close to the input terminals.

One question I have ... if anyone here can shed light on the subject:
What about Bi-amping in this scenario?
Is there any concern with a power amplifier "seeing" only the mid and HF section of the crossover? Will that section alone present a difficult impedance load?

I begin in earnest the project very early in the New Year. I booked an overdue 9 days time owing, away from work for this.
What better way to spend some Canadian winter !? _________________Music is food for the soul.

An Update .....
I have completely refinished the cabinets now, at least the wooden sides.
I elected to leave the original vinyl cladding on top and rear. It was (is) in good condition.

The chosen procedures employed below were dictated by A. the need to work in an unventilated basement, mid-winter, and B. with a SWMBO wonderful lady with a particular sensitivity to anything of a petroleum based.

I sanded by hand all the original finish from the panels down to the bare wood. Easier than I expected it to be, this task surprised me with it's simplicity. The original coatings simply floated on top, there was no colour stain in the wood at all.
I used a water based wood dye in a deep rich tone called Merlot (and I do love my Merlot!). 2 coats was sufficient to give superb depth and tone.
Next went on 4 coats of premium Shellac, mixed from flakes. This was awkward, as my love of Merlot does NOT extend to 100 proof Vodka and that's exactly what Shellac dissolved in DE-natured alcohol is like in aroma. The house smelled like a distillery for the 5 full days it took to apply and sand between each coat of this amazing material.

I'm now in the process of rubbing in several coats of Liberon Black Bison wax.

As for the electronics, this was simple enough and the results are overall quite satisfying. I elected to place the bass inductor/capacitor on a small wooden board located on the bass cabinet wall, close to the dual binding post connector terminal.
The Falcon supplied board was easy to place in it's original spot on the baffle next to the T-27/ Cole's super tweeter. Wiring required some changes obviously, so I threaded a new cable from input to crossover through small holes drilled in the mid-range section. Existing wiring was used where possible for C/O to driver connections.

Stuffing was an expensive challenge. I found locally (at Toronto's premiere wool shop) a wool that very closely matches the sample piece I was able to salvage from deep within the pipe. The selected fleece comes from a Karakul beast, resident of the UK
The texture is very close to original, though I suspect the hair length is somewhat longer. This however made "fluffing" very easy and the finished stuffed cabinet is quite full and secure. I won't worry about settling in my remaining lifetime. For those who are interested I calculated carefully the quantity for each section according to Dr. Bailey's recommendation of .0425 lbs per cubic foot. This seems to be perfect sonically, I have no desire to alter this most worrisome part of the project.

There are small challenges that I'll go into detail in a later post if there's any interest here. Let me know if there is and I'll elaborate.

For now I'm exceedingly happy with the efforts results. NOT IMF 80's, but I'm pleased enough to put them in the exalted main room space they occupy for an extended relationship development.

If someone can suggest a way to post pictures (I've never been able to figure that out here) then I can give a photo play by play through the work._________________Music is food for the soul.

Well .... hmmm.
This project has become an expensive disappointment in many ways.

I am completely unable to "tune" this LT line into any semblance of a TL, or anything like described in the literature on the TL subject.
The very best I can get is a Cantata like closed box impedance graph from any type of bass loading attempts tried so far.
Fully stuffed as per spec ... it's a closed box.
Read on.

The bass in room sound is far off what I would expect from any TL design. In fact, it's muddier and less articulate than the Cantata's in my collection. There's Zero Oompf. Nothing beyond a hint of real depth, and to get that hint requires an ear up close (inches away) to the line termination.

There's a great huge +25 ohm spike at bass driver resonance (38Hz in my case) and a very similar compared to my Kef Cantata's somewhat loopy impedance above that.
The mid's sound pretty good though (if a bit more forward than I'd like), and the roll off to the T-27 is relatively smooth most of the time ... but there's no real "magic" to the presentation that keeps me enthralled. In fact on some music it's more than a bit confused and ragged. And, This is after the expensive Falcon upgrade with the 18dB slope mid to tweeter crossover too.
The IMF's have mid / HF magic in spades BTW, and I'm beginning to suspect it's perhaps the contribution of the Celestion drivers, though I hate to admit it.

In my scan's, there's no hint of any double impedance peak in the bass region to suggest a reflex type loading. Nor is there the expected smooth curve up to driver resonance with only a slight peak as suggested there should be by traditional TL stuffed line design.

It's the bass that disappoints the most.
What seems very odd to me is I get zero difference, measured or heard after unloading all wool stuffing from the bass chamber behind the B-139 driver. But I do get an extremely resonant rear panel!!! And this can't be indicative of "a good thing" can it?

Note: I have the benefit of being able to compare to the impedance graphs taken from some marvelous IMF tls80's here and recapped and reconditioned (properly ) Kef Cantata's. Both of these are of similar vintage to the 1976 TL200's.
The Cantata's are very close in impedance character to the TL-200's. But, The IMF's present an impedance sweep curve with a giant +30dB peak at very low 20Hz, and another equal peak at driver (or cabinet) resonance just under 40Hz.
Yes, I know some will say this is indicative a reflex loading, (Andy?) but it's amazingly deep, clean and clear bass. Unlike any reflex design I've ever heard, ported or passive radiator loaded. This is the sound I've grown to love, and this is what I'd expect (or near) to any well designed TL .. including the Cambridge.

So ... I'm very open to suggestions here if anyone cares to jump in with something to make me want to continue the project. Otherwise I'm going to eventually throw in the towel on these and do what I loath to do.... Part them out and make space for something that satisfies.
Sad. Very very sad.

Is it possible that the line is not the problem. Have you checked that B139īs are working perfectly... you should have plenty of them to compare. Is crossover LF section connected correctly, cap and choke reversed will kill LF. Is phasing of drive units OK... Are there any air leaks...

Mistakes like these seem to happen to me sometimes... and something so basic like air leak around drive unit is sometimes difficult to remove (at least without original gasket or silicone sealer).

Thanks for the uplifting encouragement. But I'm pretty near certain the line is the main difficulty.
Of course, in all my studying up on the subject of TL design that point was well forewarned! The biggest challenge is the tuning.

I have a collection of 6 spare relatively good condition B-139's to choose from, though none of them show a resonance below 32Hz, and the best ones were installed. The originals to this set of TL-200's read a free air resonance of 38Hz.

The simple Cap / Inductor of the 12 dB slope Falcon crossover was mounted on a separate board located in the bass section behind the woofer to facilitate bi-wiring. It is wired correctly.

I did not have the benefit of getting these speakers with their original stuffing intact. This necessitated sourcing new long-haired wool and stuffing according to Arthur Bailey's formula of .452 Lb's per cubic foot of line.
Note, I only have access to the space immediately behind the woofer and the pipe up the rear towards the first turn. From what I can see the original stuffing remains in the last part of the line unaccessible to me. I can only guess at it's density there.

Further study indicates that an impedance sweep showing a single large peak at driver resonance indicates an under-stuffed line, so I suppose I'll start there ... experimenting with a larger mass of fill. However, the line sure seems pretty full as is!

Another point of concern is the original B-110a's.
Both show a dcr of a very low 6.6 ohms. (On their way to B-110 heaven?) This undoubtedly results in a shift of the intended crossover point and quite possibly messes up the levels. I'm considering swapping those out for some "to-spec" sp1057 B-110b's. The only big difference I can see in the specifications between the 2 versions is the sensitivity, the 110b's are more efficient. Worth a try though I suppose.

Not easy to work on, these beasties weigh in at 100 Lb's each. Getting them up on crates on their backs and the baffles off (all 14 screws each) is a workout in itself.

Regardless .. I'm in for the long haul, committed to doing the absolute best possible.
I'll keep the post up to date for benefit of current or future readers._________________Music is food for the soul.

I am completely unable to "tune" this LT line into any semblance of a TL, or anything like ...

If you can't tune a box with a hole in it, such that you get two impedance peaks, then what you have is a very lossy closed box (a box with a leak). If you want to utilise a resonance, be it Helmholtz or pipe, then you will have to reduce the losses, i.e. remove some lining. There is no magic or mystery here.

I should also add, if the impedance in the box is not much different to the woofer in free air, then you don't have a box, just a collection of pieces of wood!

Next round of stuffing and adjusting .... I have lined the bass cabinet walls with convoluted foam 1.25" thick.
I have retained the original weight of wool stuffing in both the pipe and the bass cabinet.
Viola! I have the start of a TL expected impedance sweep result, with a slight (15ohm) peak at 12Hz, a gentle dip down to 10 ohms to 20Hz where it begins a steep rise to 35 ohms at 40Hz.
This is progress! Not there yet obviously, but it does seem that extra stuffing is heading things in the right direction.
(Physics be damned SpeakerGuru ?! Blasphemy, I know.)

I have also swapped out the original B-110 for a B-110B sp1057. This will take some A/B comparing over a bit of time before giving an opinion, but it has dramatically smoothed out the impedance line.
It's gentle roll, 6 to 10 Ohms between 100Hz to 20KHz. Between that range there's only 2 slight (2 ohm) peaks at 300Hz & 1KHz,

And, I hear no obvious change in output level between the 2 mid-range drivers. So I suppose I've answered my own question posted in another thread ... perhaps it's fair to say the Sp-1003 / Sp-1057 are indeed interchangeable (under certain circumstances at least, like when your 1003 is going south).

As soon as my back recovers I think I'll have another go at stuffing adjustments.

I should have waited for retirement though ... this is Time Consuming!_________________Music is food for the soul.

Not sure if anyone's listening ... but for posterity sake here's another update.
No .. these won't "tune up" to a true TL type response. Not yet anyway. Speakerguru, you're right. I have a Lossy box. Only. But it's getting much more satisfying to listen to at least.

Not being completely happy with the extra foam addition to the bass section walls I removed it and made one crucial change that DOES work very well sonically.
I have replaced the 12dB crossover bass section with a 3rd order filter rescued from some KEF Cantata's. I can see (hear) why this is THE recommended filter arrangement for the B-139.
Never have I seen such a steep roll off filter used in the bass section of any TL though. They always seem to be a lower 12dB.

As noted previously I have changed the B-110a's for B-110b's (sp-1057) and these will definitely stay. There's a world of difference and to quote a phrase ... "It's All Good".
Next step is to modify the Falcon crossovers with some series resistance at the input stage of the B-110 driver (ala Cantata). Question is only, How Much, What Power Rating, and What Type of resistor. Any suggestions ??????
The plan is to pad down the mid range hopefully enough to make the bass a bit more satisfying still.

That bass by the way still shows an impedance curve more indicative of a closed box than a TL. Extra stuffing's been added at the port end of the pipe and yes there's some very very low frequency output audible from it. Comparable to the IMF's but not what it should be for sure. Regardless, it's at least as good as closed box Cantata bass ... better I'd say in fact, just not as loud !!! Frustrating.

I feel that the extra 0R7 of the Bass filter has likely compromised the level balance .... and I'd love to get that bass output up a bit. Hopefully the mid range padding effects that.

As always ... I'm wide open to suggestions._________________Music is food for the soul.

I have just acquired 2 pairs of Cambridge Audio TL-200 speakers in mint condition from the original owner. I don't find a whole lot of information online about these speakers. Does anyone know how many were produced? How do you know when it's time to replace crossovers? What replacement crossovers do you recommend? I'm thinking about replacing the speaker wire screws with something I can plug in. Any thoughts there?